What we call "yams" here in America are, in fact, sweet potatoes.
Yams are not sweet potatoes. Similar but still 2 different things.
Yams are indigenous to the Carribean and the African and Asian continents.
In the Southern states, they may call the sweet potato a yam, but they are not.
And to add to the confusion, the cooked, canned sweet potatoes are also not yams, even though it says so on the can label.